Mixtures of waste materials containing desirable plastics and other materials are often processed to recover either or both of the plastic and other components. Such methods include burning the mixture to remove the plastic component, dissolving the plastic component in a chemical solvent to recover the non-plastic and a solution of the plastic component, electrostatic separation, and separating the plastic component from the non-plastic component by a flotation process.
In those methods based upon the flotation process, described for example in U.S. Pat. Nos. 3,670,969, 4,000,031, and 4,352,732, it is known to charge a plastic and non-plastic containing scrap to a liquid bath having a specific gravity intermediate the specific gravities of the plastic and non-plastic components. The plastic floats on the surface of the bath, while the non-plastic sinks to the bottom, thereby facilitating the removal and recovery of the plastic.
Frequently, density modifiers are used to adjust the specific gravity of the bath, in order to float plastics having a greater specific gravity than the pure bath liquid. U.S. Pat. Nos. 4,578,184 and 4,728,045 generally disclose the use of a water bath which may be density-modified by the addition of salts to form an aqueous solution. A solution, such as sodium chloride dissolved in water, is a physically homogeneous mixture of two or more substances, wherein the constituents cannot be separated from each other by mechanical means. However, salts and other solutes have a corroding effect on metals, and are accordingly not useful for the processing of mixtures containing metal as the non-plastic component, such as for example the separation and recovery of plastics and metal from insulated wire and cable scrap. Moreover, salts and other solutes form a detrimental residue on the recovered plastic components, which must be removed prior to reprocessing the reclaimed plastic.
U.S. Pat. No. 4,119,533 discloses the separation of two or more plastics based upon their different hydrophobicities, by immersion into an aqueous medium which may be density-modified by the addition of salts to form a solution. This technique is quite different from the separation of plastics by flotation based upon their different specific gravities.
U.S. Pat. No. 4,746,422 discloses the separation and recovery of plastics from a contaminant, by a flotation process employing a dual phase mixture of water and a hydrocarbon. The suggested hydrocarbons are costly, and pose environmental and employee safety risks.
U.S. Pat. No. 3,516,841 discloses a method for the recovery of plastic from a metallic-plastic laminate, employing a density-modified aqueous solution flotation process. In the event that the mixture contains aluminum, mercuric chloride is added to the aqueous solution, to convert any aluminum metal retained on the recovered plastic to aluminum hydroxide. The aluminum hydroxide is thereafter converted to aluminum oxide by drying the recovered plastic. Aluminum oxide is identified as a beneficial impurity in the recovered plastic, as it provides a "filler" for subsequent processing. The disclosed process is useful for the conversion of thin layers of aluminum to aluminum hydroxide by the action of mercuric chloride, but is not amenable to the complete conversion of the aluminum core of wire or cable scrap to aluminum hydroxide. Furthermore, such a process would not be useful for processing wire or cable scrap having core metals other than aluminum. Moreover, mercuric chloride is toxic by ingestion or absorption. Finally, the density modifying salts used to produce the aqueous solution are detrimental and therefore require removal before reprocessing the recovered plastic.
It must be noted that the prior art referred to herein above has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention.
It would be desirable to develop a process for separating and recovering plastic from mixtures containing the desired plastic and other materials, which would not utilize corrosive solute or salt solutions, hydrocarbons, nor toxic substances such as mercuric chloride. Additionally, such a process would not require post cleaning of solutes or salts from the reclaimed plastic prior to its reprocessing.